The presence of slender or clearly defined incisions on the surface or within the depths of concrete elements serves as a visible sign of cracks. These cracks can be classified into two categories: structural and non-structural. In this article, we will explore the specifics of cracks in concrete beams.
Cracks in Concrete Beams: –
Cracks observed in concrete beams can signal potential structural failures resulting from either deficient design or faulty construction practices. Such issues may arise from failures within the beam itself or be influenced by various external factors. One notable example involves the impact of different loads on a column, such as shear forces that induce shear stress. This stress tends to peak at the joint between the column and the beam.
In instances where the maximum shear stress exceeds the limiting shear stress of the beam (which represents the beam’s strength), it can lead to the formation of cracks on the lower side of the beam. Identifying and understanding these factors is crucial for addressing and rectifying structural concerns in concrete beams.
Also, read: Types of RCC Beam
Types of Cracks in Concrete Beams: –
Concrete beams may exhibit various types of cracks, each attributed to specific causative factors. Identifying these cracks is crucial for targeted analysis and effective remediation. The following categories of the types of cracks observed in concrete beams:
- Share cracks
- Flexural or tensile cracks
- Compression failure cracks
- Cracks due to corrosion or insufficient concrete cover
- Shrinkage Cracks in concrete beams
- Sliding Cracks in concrete beams
Share Cracks:
Shear forces exerted on a concrete beam induce shear stress, particularly concentrated in proximity to the joints or supports such as columns and walls. This stress becomes most pronounced near the beam’s ends. When the magnitude of shear stress surpasses the beam’s strength, commonly referred to as its limiting stress, it leads to the development of cracks in the concrete beam, primarily near the support or at the beam’s extremities.
It is noteworthy that shear stress is minimal at the midspan of the beam, precisely at the point L/2. Consequently, cracks do not manifest at this location. The fractures that emerge near the supports or beam ends are known as shear cracks, characterized by a 45-degree inclination with the horizontal plane.
Also, read: Types of Beam: Materials for Construction, Support Types
Important Characteristics
Appears at the end of the beams at an angle of 45 degrees in plain. There is no appearance of cracks in the mid-span of the beams
Possible Reasons
Due to the application of increased shear stress on the beam.
Flexural or Tensile Cracks:
The term “flexure” denotes the action of “bending.” When external forces act upon a concrete beam, it undergoes a bending moment, resulting from the bending of the beam. This bending phenomenon gives rise to two distinct types of bending moments: internal and external.
The developed bending moment can be categorized into two forms – positive bending moment, also known as sagging, and negative bending moment, termed hogging. A positive bending moment occurs when the beam is subject to an upward force at its midspan, leading to tensile forces in this region. Consequently, the increased positive bending moment prompts the development of tensile stresses in the midspan, ultimately causing the formation of cracks at this location, as illustrated in Figure 2.
Similarly, when there is an augmentation of negative bending moment, particularly at the ends of the concrete beam, tensile forces manifest on the side of the beam facing away from the applied load. This increase in negative bending moment leads to the development of tensile stresses at the beam’s ends. Consequently, cracks emerge at these locations due to the tensile forces exerted, illustrating the impact of negative bending moment on the structural integrity of the beam.
Important Characteristics:
Originating in the region of the maximum moment (as seen in the image, typically at the centre of the beam, with variations depending on the support conditions of the beam), these cracks may appear singularly or in clusters. They exhibit their maximum width either at the bottom or the top of the beam.
Possible Reasons:
The flexural capacity of the beam is inadequate. When the Cross section of the beam or main reinforcement in the beam is insufficient it is likely to develop cracks.
Compression Failure Cracks
When the beam experiences the compressive force it tends to sag.
Important Characteristics:
Originates nearer to maximum torsion region. Single generally uniform width. Appears over the whole periphery in helical form
Possible Reasons:
The torsional strength of the beam is inadequate. Cross-section of torsional reinforcement insufficient
Cracks Due to Corrosion or Insufficient Concrete Cover
Corrosion cracks in reinforced concrete beams run along the line of reinforcement. It usually separates the concrete from reinforcing bars. It is mostly manifested by discolouration of paint or stains of rust.
Important Characteristics
Runs along the line of reinforcement. Uniform width in general
Possible Reasons
The bond between reinforcing bars and concrete is not satisfactory. May be due to corrosion of bars/fire damage.
Shrinkage Cracks in Reinforced Concrete Beams
Shrinkage cracks in reinforced concrete beams occur during two stages, which are the pre-hardening stage and the hardened stage. In the pre-hardening stage, these types of cracks are called plastic shrinkage cracks & in the hardened stage, they are known as drying shrinkage cracks. Shrinkage cracks occur when fresh concrete is subjected to a very rapid loss of moisture.
Important Characteristics:
No regular pattern or thickness and in general superficial.
Possible Reasons:
Curing is inadequate or has no control over the water-cement ratio. Usage of an excessively rich mix. Shrinkage reinforcement, if any, is insufficient.
Also, read: Development Length for Reinforcement Bar: Anchorage Length & Lap Length with Formula
Sliding Cracks in Reinforced Concrete Beams
The diagonal mode of failure by sliding along the critical cracks is known as failure by sliding and usually appears at the edge of the supports of the beam. These types of concrete cracks appear if the concrete gets disturbed in a fresh state.
Also, read: Column Failure: Causes, Types and Prevention
References:
- Bureau of Indian Standards. (1984). Handbook on Causes and Prevention of Cracks in Buildings (SP 25). BIS. https://pwd.portal.gov.bd/sites/default/files/files/pwd.portal.gov.bd/page/0ef2c66e_9492_4035_9235_3931ed772445/Hand%20Book%20on%20Causes%20and%20Prevention%20of%20Carcks%20in%20Buildings.pdf
- Bureau of Indian Standards. (2000). Plain and Reinforce Cement Concrete (IS 456).
- Chandra, R. (2013). Reinforced Concrete Structure (Limit State Design) (1st ed.). Standard Book House, Rajsons Publications Pvt. Ltd.
